1. Field of the Invention
This invention relates generally to modulator driver circuits and more particularly to an optical modulator with a driver output stage with low power dissipation.
2. Description of the Related Art
It is common practice in a drive circuit to employ transistor circuitry to provide a signal driver for modulating an electro-optical modulator, such as a Mach Zehnder modulator (MZM) or a semiconductor electro-absorption modulator (EAM). A typical driver circuit for an EAM is shown on U.S. Pat. No. 5,706,117 in FIG. 15, for example, as well as U.S. Pat. No. 6,014,392. The latter mentioned patent deals with wavelength dispersion of the EAM which is also discussed in U.S. Pat. No. 5,917,637. The output stage of such an optical modulator driver as known in the art is also illustrated in FIG. 1. In FIG. 1, the signal source shown as driver amplifier 22 controls the peak to peak amplitude of the transition bits representing the signal bits. The drive circuit 10 employs a differential transistor pair arrangement 11, also referred to as an emitter coupled differential circuit to provide current switching operation, which steers or directs the DC current to or away from optical modulator 12. Modulator 12 is shown here as a EAM. Because an optical modulator responds to voltage, and not to current, the drive current must be converted to a voltage. A load resistor is added in parallel with modulator 12, as shown in U.S. Pat. No. 5,706,117, to convert the drive current into a voltage. The power dissipation of circuit 10 is equal to the amount of the DC current, ID, in drive circuit 10 times the DC supply voltage, VD.
In the case of FIG. 1, there are two resistors 14 and 16 that are necessary to terminate the differential circuit. Only the voltage across resistor 14 is employed to drive modulator 12. This impedance across or in parallel with modulator 12 is generally designed to be a 50 ohm load, as indicated in FIG. 1, from which impedance matching is achieved relative to RF transmission lines 18 and 20. The resistance of resistors 14 and 16 is usually low, such as on the order of 50 xcexa9, in order to match its resistance to the low impedance RF transmission lines 18 and 20 which electrically couple the RF drive signal from the output driver stage 10 to EAM 12. The resistance of resistor 14 is also usually low in order that the modulator time constant, determined by the product of the optical modulator parasitic capacitance and its resistance, is low. As a result, the drive current, ID, will be high resulting in high power dissipation. As an example, for 2.5 V amplitude and 25 ohm load, the current could be 100 mA, for a supply voltage, VDD, about 5.2 V. This type of drive circuit 10 nominally consumes, for example, approximately 0.5 W of power.
One alternative approach is shown in the article of Kai-Yap Toh et al. entitled, xe2x80x9cA 23-ps/2.1 mW ECL Gate with an AC-Coupled Active Pull-Down Emitter-Follower Stagexe2x80x9d, IEEE Journal of Solid-State Circuits (Vol. 4(5), pp. 1301-1306, October, 1989. See, for example, the circuit in FIG. 5 of this article which comprises an emitter-coupled logic (ECL) circuit consisting of an emitter coupled differential circuit in combination with a xe2x80x9ctotem polexe2x80x9d circuit comprising a coupled emitter-follower pull-up transistor and a coupled emitter follower pull-down transistor, including a bias current circuit for the node between the control capacitor and the base of the pull-down transistor. A substantially same circuit is also shown in FIG. 13 and described in U.S. Pat. No. 5,574,391, which patent represents an improved upon circuit over the FIG. 13 ECL circuit by eliminating the bias current circuit to provide for even further reduced power consumption through the utilization of current mirror transistor circuit arrangement.
Still, the power consumption is higher than desired for many applications, particularly the application of interest here which is the modulation of optical modulators. In this connection, none of the foregoing emitter-coupled logic circuits deal with the issue of the deployment of such drive circuits in an optical modulator environment, such as a semiconductor EAM that requires high frequency modulation, such as 10 GHz or 40 GHz signals, to be applied via RF transmissions 18 and 20. Another example of a totem pole bootstrap driver circuit used in telecommunications is illustrated in U.S. Pat. No. 6,002,269.
It is desirable that such high power dissipation in such drive circuits for high speed optical modulators be significantly reduced without the need for an RF transmission line and additional power consumption due to utilization of a pull-down transistor bias current circuit. This becomes particularly so where these drive circuits become integrated into packages containing photonic integrated circuit (PIC) chips, or even integrated as part of such a PIC chip in order that power and heat consumption can be minimized reducing both the overall power and dissipated heat budget of such packages and chips.
Therefore, it is an object of the present invention to overcome the aforementioned problem.
According to this invention, an optical modulator drive circuit provides a different approach to driving a optical modulator than presently employed, resulting in a reduction in power dissipation of the drive circuit by as much as about 80%. Therefore, this invention dissipates as little as 20% of power of the present drive circuits known in the art. The optical modulator may be a semiconductor electro-absorption modulator but the principle of the invention can be applied any other type of electro-optic modulator that relies on a voltage to modulate an optical signal.
The optical modulator is driven by an ECL circuit deployed in a xe2x80x9ctotem polexe2x80x9d or push-pull arrangement to apply a low-impedance voltage source to the optical modulator. No resistor is necessary in parallel with the modulator to provide the voltage to the optical modulator. The invention provides a relatively small current to charge and discharge the modulator capacitance, and is provided only during the bit transitions in the modulation of the signal. Outside of the modulation bit transitions, the current is automatically reduced even further. As a result, the output stage drive circuit nominally consumes much lower power by as much as 80%, which is approximately 20% of the prior art approaches, for example, power consumption in the range of about 0.1 W to about 0.20 W of power. This is of high importance where these circuits are deployed in connection with optical transmitter photonic integrated circuits (TxPICs) as illustrated in U.S. patent applications, Ser. No. 10/370,345 and 10/267,331, both filed on Oct. 8, 2002, which applications are incorporated herein by their reference. In this kind of optical integrated chip environment where there is an array of modulators integrated in a single semiconductor chip with a laser diode (LD) array, such as a DFB array, and an optical multiplexer, such as an AWG, power budget constraints become highly critical in the operation of the chip so that power consumption must be reduced to as low as possible without sacrificing high speed modulation such as at 10 GHz and 40 GHz.